1. Field of Technology
The present invention relates to an apparatus for converting a television signal which is in accordance with the MUSE (Multiple Sub-Nyquist Sampling Encoding) system to a television signal of the NTSC (National Television Systems Committee) system, for enabling pictures of the MUSE television signal to be displayed by a conventional NTSC standard television receiver.
2. Prior Art Technology
The MUSE television system has been developed in order to enable a high-definition television signal to be bandwidth-compressed to an 8.1 MHz bandwidth MUSE signal, which can be broadcast by transmission via a single satellite channel. The MUSE system is described in detail in "Nikkei Electronics", 1987.11.2, (pages 189 to 212), published by Nikkei McGraw-Hill Co. Since television receivers equipped for high definition television operation are not in wide use at the present time, it is desirable that a MUSE broadcast signal can be applied to a conventional television receiver and displayed. In the following it will be assumed that the conventional television receiver is of the NTSC standard. However the problem arises that there are some basic differences between the MUSE system television signal and the NTSC system signal. In particular, the number of scan lines per frame, the aspect ratio of the television picture, and the method of multiplexing the color signals within the television signal are all significantly different between the MUSE and the NTSC systems. These differences are set out in the following table.
TABLE 1 ______________________________________ MUSE NTSC ______________________________________ Number of scan 1125 525 lines per frame Effective number 1032 483 of scan lines per frame Aspect ratio 16:9 4:3 Method of color Line- Chroma signal signal sequential superimposed multiplexing TCI on luminance method signal ______________________________________
In the above, "effective number of scan lines" signifies the number of scan lines which contain picture information (as opposed to audio signals, synchronizing signals, etc.).
One method which has been proposed in the prior art for conversion from MUSE to NTSC signal is to use a vertical filter to select 1/2 of the scan lines of a frame of the MUSE signal, to form the scan lines of one frame of the NTSC signal, and to clip off portions at the right and left sides of the display (i.e. to omit the corresponding portions of the luminance data in each scan line of the MUSE signal) to thereby obtain a signal which will provide an aspect ratio of 4:3. The color data of each effective scan line of the MUSE signal (where "effective scan line" has the significance defined hereinabove) are time-expanded (to compensate for a 1/4 compression of the color data that is executed in forming the MUSE signal, with the compressed color data being inserted into the horizontal blanking intervals of respective scan lines) and then multiplexed with the luminance signal in accordance with the NTSC standard. In the MUSE system, each effective scan line contains either B-Y data for one line, or R-Y data for one line, alternating in successive lines. Thus, each frame contains complete B-Y and R-Y data for only half of the total number of effective scan lines. The basic features of this conversion method are illustrated in FIG. 1, with the C (color signal) and Y (luminance signal) components of one frame of the MUSE signal and a corresponding frame of the NTSC signal being conceptually indicated as display regions. As illustrated, the compressed C component of the MUSE signal is inserted in each scan line before the portion of the scan line which contains the Y component. In effect, the portions of the C and Y signal regions of the MUSE frame that are shown as hatched-line regions are "cut out", the cut-out C region is time-expanded and combined with the cut-out Y region (after selecting 1/2 of the total number of scan lines of the cut-out Y region, to obtain 483 effective scan lines per frame), to thereby obtain the corresponding NTSC signal region indicated as (Y+C), which will provide a television picture having an aspect ratio of 4:3 and 483 effective scan lines (with a total of 525 scan lines) per frame.
However with such a method of conversion from the MUSE to the NTSC system, the problem arises that in order to obtain the NTSC system picture, relatively large portions are clipped from the right and left sides of the MUSE picture. In many cases, important information or picture detail will be contained in these regions at the right and left sides, so that the viewer will desire to see the entire MUSE system picture, without clipping of the side regions.